Star isobutylene-based thermoplastic elastomers

ABSTRACT

A star block copolymer and a thermoplastic elastomer including plurality of the star block copolymers and a method of making both is taught. The star block copolymers of the present invention include a core component having an α-methylstyrene oligomer wherein arms emanate from the core component and the arms are poly(isobutylene-block-styrene) diblock copolymers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 62/714,817 entitled “Novel Star Isobutylene-Based ThermoplasticElastomers” filed Aug. 6, 2018, which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates to the structure and synthesis of a starblock copolymer and to the structure and synthesis of a star blockcopolymer-based thermoplastic elastomer. Most particularly, the presentinvention relates to a star block copolymer-based thermoplasticelastomer exhibiting advantageous mechanical properties such as highstrength and essentially no creep. Furthermore, the star blockcopolymer-based thermoplastic elastomer also exhibits advantageousbiocompatibility and biostability, advantageous oxidative-hydrolyticresistance, advantageous barrier properties, advantageous calcificationresistance properties, and advantageous damping properties.Specifically, the present invention relates to the structure andsynthesis of a star block copolymer consisting of a oligomeric styrenecore, such as an α-methylstyrene core, from whichpoly(isobutylene-block-styrene) diblock arms emanate, and the structureand synthesis of a star block copolymer-based thermoplastic elastomercomprising a plurality of the aforementioned star block copolymers.

BACKGROUND OF THE INVENTION

Poly(styrene-b-isobutylene-b-styrene) (SIBS) is a thermoplasticelastomer that has gained attention recently due to its high degree ofbiocompatibility. Due to its biocompatibility, SIBS has been found to beuseful for a variety of applications, such as stent coating, glaucomashunt, and tubing. This linear block copolymer has a triblock structureformed by a polyisobutylene (PIB) core sandwiched between blocks ofpolystyrene (PS). The formulation of SIBS can be tailored for differentapplications by changing the weight percentage of PS or by changing themolecular weight of the polymer chains. The hard PS blocks provide SIBSwith a glassy microstructure that enhances mechanical strength andrigidity of the material, while the PIB has a soft microstructure withincreased chain mobility that gives the polymer its elastomericproperties. The possibility of tailoring mechanical properties, togetherwith the high degree of biocompatibility, makes SIBS an ideal materialfor use in biomedical devices.

However, there is a high cost associated with making SIBS. The high cost(30-40%) of most SIBS products is largely due to the expensivebifunctional polymerization initiator needed for synthesis. Typically,that expensive bifunctional polymerization initiator is1-(tert-butyl)-3,5-bis(2-chloropropan-2-yl)benzene (abbreviated hereinas HDCCl, for hindered dicumyl chloride):

5-tert-butyl-1,3-bis(1-chloro-1-methylethyl)benzene (HDCCl)

Other initiators commonly used for the synthesis of well-definedtelechelic PIBs (synthesize by living cationic polymerization (LC⁺P) ofisobutylene) include those described in U.S. Pat. No. 5,733,998 toKennedy et al. and U.S. Pat. No. 8,889,926 to Kennedy et al., thedisclosure of which are incorporated herein by reference in theirentirety.

Block copolymers of similar compositions often have diverse mechanicalproperties due to their composite nature. Parameters such as molecularweight, block weight percentage, and polymer chain structure are knownto give rise to different microstructures that, in turn, lead todifferent material properties. Different grades of SIBS can have verydifferent morphologies based on the ratio of hard phase to soft phase.At lower contents of PS, the hard phase forms spherical domains throughthe soft matrix. As the PS content increases, the spherical domainsbecome double gyroid structures, and as the PS content is furtherincreased, the structure of the hard phase becomes lamellar. It islikely that the incompatibility of the soft and hard phases leads tomicro-phase separations and results in the different morphologiesdescribed. It is well known that for composite systems, the interfacebetween different phases plays a major role in the performance of thematerial. A weakened interface might lead to premature cracking andfailure. Additionally, the method of fabrication for SIBS might play avery important role due to the incompatibility of the different phases.Therefore, different methods may result in different qualities of theinterface.

A less expensive industrial version of a PIB-based linear thermoplasticelastomer is SIBSTAR™, commercially available from Kaneka Co. However,this product is known to be contaminated with byproducts and has worsemechanical properties than well-defined SIBS, which therefore severelylimits its use.

However, for all of its attributes, SIBS has been found to be of modeststrength and tends to exhibit higher creep than desired for manyapplications, including medical devices. Therefore, the need exists fora new PIB-based star-shaped thermoplastic elastomer, useful forimplantable medical devices and industrial applications, that have thekey advantageous properties of SIBS, such as biocompatibility,biostability, elasticity, and processability, but that also exhibitshigher strength and toughness as compared to SIBS, and diminished toessentially no creep, which SIBS does not exhibit. Furthermore, this newmaterial should be able to be synthesized without the use of a costlymulti-functional initiator.

SUMMARY OF THE INVENTION

Generally, the present invention provides a star block copolymercomprising a core component having a styrene oligomer, such as anα-methylstyrene oligomer, and arms emanating from the core componentwherein the arms are poly(isobutylene-block-styrene) diblock copolymers.Furthermore, the present invention provides for the formation of athermoplastic elastomer through the physical crosslinking of a pluralityof the star block copolymers.

Advantageously, it will be appreciated that the star block copolymers ofthe present invention have a tensile strength of greater than 20 MPa,preferably greater than 24 MPa, and more preferably, greater than 28MPa, and have essentially no creep deformation while being cheaper andeasier to process than other block copolymers, such as SIBS. By“essentially no creep” or “essentially no creep deformation,” it ismeant that any creep of the copolymers and thermoplastic elastomers ofthe present invention is de minimus and does not in any way affect theessential nature of the composition.

In one or more embodiments, the arms of the star block copolymers of thepresent invention include a polyisobutylene block having a numberaverage molecular weight of between about 30,000 and about 40,000 g/moland a polystyrene block having a number average molecular weight ofbetween about 12,000 and about 14,000 g/mol.

In one or more embodiments of the present invention, the star blockcopolymer comprises a core component having an α-methylstyrene oligomerand arms emanating from the core component, wherein the arms arepoly(isobutylene-block-styrene) diblock copolymers. In at least oneembodiment, the start block copolymer is produced by synthesizing aα-methylstyrene oligomer; acetylating the α-methylstyrene oligomer toform an α-methylstyrene oligomer with acetyl groups; converting theacetyl groups to cumyl hydroxide groups; undertaking livingcarbocationic polymerization of isobutylene to form polyisobutyleneblocks; and undertaking living carbocationic polymerization of styreneto form polystyrene blocks at an end of each polyisobutylene block toprovide the poly(isobutylene-block-styrene) diblock copolymer arms.

In a further embodiment, the present invention provides a method ofsynthesizing a thermoplastic elastomer containing a plurality of starblock copolymers comprising a core component having an α-methylstyreneoligomer and arms emanating from the core component, wherein the armsare poly(isobutylene-block-styrene) diblock copolymers. The methodincludes the formation of a plurality of the star block copolymers asdiscussed above, and then physically crosslinking through aggregation ofthe polystyrene portion of the arms of the plurality of star blockcopolymers so as to form the thermoplastic elastomer.

Advantageously, the present invention provides a star blockcopolymer-based thermoplastic elastomer exhibiting advantageousmechanical properties such as high strength and essentially no creep,advantageous biocompatibility and biostability, advantageousoxidative-hydrolytic resistance, advantageous barrier properties,advantageous calcification resistance properties, and advantageousdamping properties.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the features and advantages of thepresent invention, reference is now made to the detailed description ofthe invention along with the accompanying figures in which:

FIG. 1 is a synthetic strategy for the preparation of the star blockcopolymer of the present invention;

FIG. 2 is a synthetic strategy for the preparation of the core componentof the star block copolymer of the present invention;

FIG. 3 is a representative microarchitecture of a representative starblock copolymer of the present invention; and

FIG. 4 is a representative microarchitecture of a representativethermoplastic elastomer of the present invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The present invention generally relates to a thermoplastic elastomerhaving improved mechanical properties over ordinary thermoplasticelastomers. It will be appreciated that the generally recognizedunderstanding of the term “thermoplastic elastomer” refers to the classof copolymers which consist of materials with both thermoplastic andelastomeric properties. Thermoplastic elastomers show advantages typicalof both rubbery materials and plastic materials.

As use herein, the term “thermoplastic elastomer” will also be definedas meaning a plurality of star block copolymers physically crosslinkedwith one another and wherein the star block copolymers comprise a corecomponent having a styrene oligomer, such as an α-methylstyreneoligomer, and arms emanating from the core component wherein the armsare poly(isobutylene-block-styrene) diblock copolymers.

Generally, the thermoplastic elastomers of the present invention includea plurality of star block copolymers each consisting of a core componenthaving a styrene oligomer, such as an α-methylstyrene oligomerabbreviated hereinafter as OαMeSt (where the O stands for oligomeric),and arms emanating from the core component, wherein the arms arepoly(isobutylene-block-styrene) diblock copolymers, abbreviatedhereinafter as P(IB-b-St). Throughout the course of this disclosure, thestar block copolymer of the present invention, consisting of a corecomponent having at least a styrene-based oligomer, and preferably, anα-methylstyrene oligomer, and arms emanating from the core componentwherein the arms are poly(isobutylene-block-styrene) diblock copolymers,will be abbreviated as OαMeSt-g-P(IB-b-St).

In one or more embodiments, the OαMeSt-g-P(IB-b-St) of the presentinvention is represented by the following formula:

It is understood by those skilled in the art that the range 520-820 isan approximation of the number of isobutylene units in the arm and thatthe range 120-140 is an approximation of the number of styrene units inthe arm. These ranges of units amount to a number average molecularweight of each polyisobutylene block of eachpoly(isobutylene-block-styrene) diblock copolymer arm of between about30,000 and about 40,000 g/mol and wherein the number average molecularweight of each polystyrene block of each poly(isobutylene-block-styrene)diblock copolymer arm of between about 12,000 and about 14,000 g/mol.Due to the large size of each diblock arm, the contribution of the corecomponent, OαMeSt, to the overall weight of the OαMeSt-g-P(IB-b-St) isinsignificant at about 0.02% of the overall weight.

The structural details of the OαMeSt-g-P(IB-b-St), i.e., the length andmolecular weight distribution of the core component, OαMeSt, and thediblock arms, P(IB-b-St), the relative volume amounts of the rubberypolyisobutylene block and glassy polystyrene block, and the overall twophase morphology must be precisely controlled so as to obtain superiorproperties once combined to form a thermoplastic elastomer and to havethe OαMeSt-g-P(IB-b-St) be easy to process.

In one embodiment of the present invention, the OαMeSt core may be ahexamer. Preferably, every αMeSt unit carries onepoly(isobutylene-block-styrene) diblock arm in the star block copolymer,OαMeSt-g-P(IB-b-St). As stated above, the average molecular weight ofeach polyisobutylene block of each poly(isobutylene-block-styrene)diblock copolymer arm is between about 30,000 and about 40,000 g/mol andwherein the number average molecular weight of each polystyrene block ofeach poly(isobutylene-block-styrene) diblock copolymer arm is betweenabout 12,000 and about 14,000 g/mol. The molecular weights are dictatedby the targeted spherical morphology of a thermoplastic elastomer madefrom a plurality of the OαMeSt-g-P(IB-b-St) star block copolymers, whicharises when the hard (PSt)/soft (PIB) segment weight ratio is about30/70. The molecular weight dispersity of the PIB and PSt segments isnarrow, substantially less than about 2.0, which arises because eachblock of the diblock arm is produced by a living carbocationicpolymerization technique.

The core component, OαMeSt, provides a rigid center for the star blockcopolymer. As stated above, the preferred degree of polymerization (DP)of the core component is 6, its DP can also be 3, 4, 5, 7, 8, or higher.Importantly, the OαMeSt core component can be easily acetylated, and thepara acetyl (CH₃CO—) groups can then be easily converted to p-(CH₃)₂OHgroups, which are needed to initiate the living carbocationicpolymerization of isobutylene followed by the living carbocationicpolymerization of styrene in producing the star block copolymer.

The long rubbery polyisobutylene (PIB) block of each diblock armprovides the continuous phase which provides for high elongation andstrength, outstanding barrier and damping properties, oxidative andhydrolytic resistance, calcification resistance, biocompatibility andbiostability. The average number molecular weight of the glassypolystyrene (PSt) block of each diblock arm ensures good phaseseparation between the glassy polystyrene block and the rubberypolyisobutylene block and also yields the highest glass transitiontemperature (the glass transition temperature of polystyrene increaseswith increased molecular weight and reaches about 93° C. at about 10,000g/mol). The polystyrene (PSt) block also provides the ability tophysically crosslink with other polystyrene blocks of otherOαMeSt-g-P(IB-b-St) star block copolymers when combined to form athermoplastic elastomer.

The OαMeSt-g-P(IB-b-St) star block copolymers of the present inventionhave a tensile strength of greater than 20 MPa and have essentially nocreep deformation. These numbers are a large increase when compared toSIBS which exhibits modest tensile strength numbers of less than 20 MPaand poor creep resistance. On account of its star shape, theOαMeSt-g-P(IB-b-St) star block copolymers of the present invention is ofrelatively lower viscosity than SIBS and can therefore be easier meltprocessed. Furthermore, the radius of gyration of theOαMeSt-g-P(IB-b-St) star block copolymers of the present invention islower than that of SIBS, which results in lower viscosity andconsequently easier processing. Additionally, the OαMeSt-g-P(IB-b-St)star block copolymers of the present invention are soluble in organicsolvents, so they can also be solution processed. Thus, theOαMeSt-g-P(IB-b-St) star block copolymers of the present invention canbe used in the medical field as well as in a multitude of differentindustrial and consumer applications such as hot melt and pressuresensitive adhesives, overmolds, electrical insulators, automotive uses,sealants, caulks, damping devices, and tubing.

The first step in producing the star block copolymer comprising a corecomponent having an α-methylstyrene oligomer and arms emanating from thecore component wherein the arms are poly(isobutylene-block-styrene)diblock copolymers is to synthesize the core component.

FIG. 2 shows a synthetic path for synthesizing the α-methylstyreneoligomer. Oligomerization is carried out under high vacuum conditions.The α-methylstyrene monomer is freshly distilled over calcium hydride orother desiccant under reduced pressure and it is then distilled overdibutylmagnesium or similar reagent under high vacuum conditions. Next,oligomerization is initiated when n-BuLi is added to the monomer in asolution of THF under strong stirring at a temperature of between −80and −75° C.

Oligomerization is conducted for a period of between about 4 and 6 hoursand it may be terminated by the addition of any known termination agent,including methanol. The product formed is then precipitated into excessmethanol, filtered and dried under reduced pressure.

Once the α-methylstyrene oligomer is formed, it is then acetylated. Tobegin acetylation of the oligomer, anhydrous AlCl₃ and CH₂Cl₂ are addedtogether, and this mixture is then cooled to about 0° C. CH₃COCl issubsequently added dropwise over a 10 minute period. Next, theα-methylstyrene oligomer is added to the reaction mixture, and thereaction is allowed to proceed at room temperature.

After acetylation of the α-methylstyrene oligomer, the acetyl groups areconverted to cumyl hydroxide groups. In this process, anhydrous diethylether is added to a solution of methyl magnesium bromide in a reactingflask. Concurrently, the α-methylstyrene oligomer with acetyl groups isdissolved in anhydrous diethyl ether. This solution is then added to thereacting flask and the reaction is allowed to proceed. After thereaction proceeds, concentrated H₂SO₄ is poured onto ice and then theicy solution is diluted with distilled water and subsequently added tothe reacting flask.

Upon formation of the α-methylstyrene oligomer with cumyl hydroxidegroups, living carbocationic polymerization of isobutylene is undertakento form the polyisobutylene blocks. To conduct living carbocationicpolymerization, the core component, i.e., the OαMeSt product with cumylhydroxide groups prepared above, is added to a round bottom flaskequipped with a magnetic stirrer, and the flask is then evacuated for 30minutes. Hexane and CH₂Cl₂ are transferred into the flask, andtetramethylethylenediamine (TMEDA) is added. The system is then cooledto a temperature of between about −85 and about −75° C., and isobutyleneis added. The solution is strongly stirred. Living carbocationicpolymerization of isobutylene is initiated by the addition of acoinitiator such as TiCl₄. The reaction is allowed to continue forapproximately 2 hours.

Upon depletion of the isobutylene monomer, the monomer is switched tostyrene. Living carbocationic polymerization of styrene is thenundertaken to form polystyrene blocks at an end of each polyisobutyleneblock, thereby providing the poly(isobutylene-block-styrene) diblockcopolymer arms. To conduct the living carbocationic polymerization ofstyrene, 2,5-di-tert-butyl pyridine (DtBP) and styrene are added atabout −80° C. to the living intermediate formed above. The livingcarbocationic polymerization of styrene is allowed to proceed for twohours and is then terminated by the addition of methanol or other knownterminating agent. The OαMeSt-g-P(IB-b-St) star block copolymer formedis insoluble in water and alcohols while being soluble in toluene, THF,and methylcyclohexane. It is colorless strong rubber with goodelongation and practically no creep deformation.

In a further embodiment, the present invention provides a method ofsynthesizing a thermoplastic elastomer containing a plurality of starblock copolymers comprising a core component comprising anα-methylstyrene oligomer (OαMeSt) and arms emanating from the corecomponent wherein the arms are poly(isobutylene-block-styrene) diblockcopolymers. The method includes the formation of a plurality of the starblock copolymers as discussed above, and then the physical crosslinkingthrough aggregation of the plurality of star block copolymers so as toform the thermoplastic elastomer.

FIG. 3 is a representative microarchitecture of a single star blockcopolymer of the present invention and FIG. 4 is a representativemicroarchitecture of a representative thermoplastic elastomer of thepresent invention. Thermoplastic elastomer 10 comprises a plurality ofOαMeSt-g-P(IB-b-St) star block copolymers 12 wherein the core componentsare represented by the number 14, the polyisobutylene blocks rerepresented by the number 16, and the polystyrene blocks are representedby the number 18. As can be clearly shown in FIG. 4, at least some ofthe polystyrene blocks 18 of the plurality of star block copolymers 12are physically crosslinked through aggregation. The microarchitecture ofthe thermoplastic elastomer 10 leads to multiple entanglements andbetter stress distribution, which results in superior strength and thevirtual absence of creep.

In a further embodiment, the present invention provides a method ofsynthesizing a thermoplastic elastomer containing a blend ofOαMeSt-g-P(IB-b-St) star block copolymers and SIBS copolymers. These twocopolymers can be easily blended together because they both containpolyisobutylene and polystyrene blocks. Thus, various proportions ofSIBS copolymers and OαMeSt-g-P(IB-b-St) star block copolymers of thepresent invention can be blended and the blends can be used to fine tunethe mechanical properties and processibility of the thermoplasticelastomer formed. These blends are homogeneous as the chemical nature ofthe polyisobutylene blocks of the SIBS copolymers and OαMeSt star blockcopolymers are identical.

In various embodiments, SIBS is blended with different amounts, frombetween about 10 and about 30 wt. % of the OαMeSt-g-P(IB-b-St) starblock copolymers. Utilizing a solution blending technique, the twocopolymers are dissolved in a common solvent, such as THF, then, thesystem is stirred for about 30 minutes, and then the blend is recoveredby precipitation into methanol and dried to a constant mass in vacuum.Alternatively, the two copolymers are melt blended in an internal mixerat a temperature of between about 165 and about 175° C. using shearrates of between about 10 and about 100 rpm. To mechanically test theblends, they are compression molded into about 0.6 mm thick films in astainless steel mold at a temperature of between about 165 and 175° C.for 15 minutes.

In a further embodiment, the present invention provides a method ofsynthesizing a thermoplastic elastomer containing a blend ofOαMeSt-g-P(IB-b-St) star block copolymers and commercially availablelinear poly(styrene-b-ethylene-b-butylene-styrene) (SEBS, sold under thetrade name Kraton™ G by Kraton Co). SEBS and the OαMeSt-g-P(IB-b-St)star block copolymers of the present invention can be blended in orderto increase the oxidation resistance and processibility of SEBS. AsKraton™ G contains between 20 and 30% by mass of polystyrene, they blendwell with the OαMeSt-g-P(IB-b-St) star block copolymers because it alsocontains polystyrene blocks.

To achieve desirable effects, Kraton™ G is blended with between about 10to about 30 wt. % of OαMeSt star block copolymers. Blending can beeffected by solution or melt blending. Thus, Kraton™ G and variousamounts of OαMeSt-g-P(IB-b-St) star block copolymers are dissolved in acommon solvent, such as THF, then, the system is stirred for about 30minutes, and then the blend is recovered by precipitation into methanol.The recovered blend is dried to a constant mass in a vacuum.Alternatively, the two copolymers are melt blended in an internal mixeroperating at a temperature of between about 165 and 175° C. under shearrates between about 10 and about 100 rpm. To mechanically test theblends, they are compression molded into about 0.6 mm thick films in astainless steel mold at a temperature of between about 165 and 175° C.for 15 minutes.

In another embodiment of the present invention, it has been discoveredthat if living carbocationic polymerization of styrene as discussedabove is allowed to proceed for more than 2 hours, preferentially forbetween about 3 and about 6 hours, a hyperbranched OαMeSt-g-P(IB-b-St)star block copolymer is formed. In one embodiment, the DtBP and styreneare added to the living intermediate formed in step four above and thenthe living carbocationic polymerization of styrene is allowed to proceedfor a time of between about 3 and about 6 hours. Subsequently, thepolymerization is terminated with methanol, the system is warmed to roomtemperature, concentrated by rotary evaporation, and precipitated intomethanol. The methanol is then decanted, the product is dissolved inhexane, and then washed with aqueous sodium bicarbonate and water. Theorganic phase is dried over magnesium sulfate overnight, the solids areseparated by filtering through fine sintered glass, and the solvent isevaporated by rotary evaporation. The product formed is calledhyperbranched OαMeSt-g-P(IB-b-St) star block copolymer. Thus, increasingthe time allowed for the living carbocationic polymerization of styreneproduces a hyperbranched OαMeSt-g-P(IB-b-St) star block copolymer havingincreased strength and modulus.

EXAMPLES

In order to demonstrate practice of the invention, the followingexamples are offered to more fully illustrate the invention, but are notto be construed as limiting the scope thereof. Further, while some ofexamples may include conclusions about the way the invention mayfunction, the inventors do not intend to be bound by those conclusions,but put them forth only as possible explanations. Moreover, unless notedby use of past tense, presentation of an example does not imply that anexperiment or procedure was, or was not, conducted, or that resultswere, or were not actually obtained. Efforts have been made to ensureaccuracy with respect to numbers used (e.g., amounts, temperature), butsome experimental errors and deviations may be present. Unless indicatedotherwise, parts are parts by weight, molecular weight is number averagemolecular weight, temperature is in degrees Centigrade, and pressure isat or near atmospheric.

The first experiment concerns the preparation of an oligomericα-methylstyrene (OαMeSt) hexamer utilizing an anionic technique. FIG. 2outlines the procedure. Oligomerization of αMeSt is carried out underhigh vacuum conditions on a conventional Schlenk line. The monomer isfreshly distilled over calcium hydride under reduced pressure and thendistilled over dibutylmagnesium under high vacuum conditions. Then,n-BuLi (12 mL, 2.6×10⁻² mol) is added to 20.26 mL, 1.56×10⁻¹ mol) αMeStin 300 mL THF under strong stirring at −78° C. Oligomerization isconducted for 5 hours and terminated by the addition of methanol. Theproduct is precipitated into excess methanol, filtered, and dried underreduced pressure. The yield is essentially quantitative. The structure,molecular weight and molecular weight distribution is analyzed by ¹H NMRspectroscopy and GPC, respectively.

In a second experiment concerns the acetylation of the OαMeSt. AnhydrousAlCl₃ (2.412 g, 18 mmol) is placed into a 250 mL flask equipped with astir bar and a 50 mL addition funnel, the flask is evacuated, and thencharged with nitrogen. Then, 30 mL CH₂Cl₂ is transferred to the flaskvia a capillary by nitrogen pressure. The mixture is then cooled to 0°C. and CH₃COCl (1.413 g, 18 mmol dissolved in 10 mL CH₂Cl₂ in theaddition funnel) is added dropwise over a 10 minute period. In order toachieve exhaustive acetylation, a 3-fold stoichiometric excess of AlCl₃and CH₃COCl are used. Then, from the first experiment, O(αMeSt)₆ (4.596g, 6 mmol) dissolved in 100 mL CH₂Cl₂ is added to the reaction mixtureand the reaction is allowed to proceed for 15 minutes at roomtemperature. The product is poured into a beaker containing about 10 gof ice and 4 mL of concentrated HCl. The organic layer is washedsuccessively with water and bicarbonate solution, separated and driedover anhydrous MgSO₄ overnight. The solution is concentrated andprecipitated into excess methanol. The yield is essentiallyquantitative. The extent of the acetylation is monitored by NMRspectroscopy.

In a third experiment, a three-necked 250 mL round-bottom flask equippedwith a stir bar, a dripping funnel fitted with a drying tube, and areflux condenser fitted with a drying tube are all through flame driedand charged with nitrogen. To the flask, 20 mL of anhydrous diethylether is added via a syringe, followed by 2.025 mL (6 mmol) of asolution of 3M methyl magnesium bromide in anhydrous diethyl ether.Concurrently, from the second experiment, 1.018 g (1 mmol) of theOαMeSt-OA_(C6) is dissolved in 20 mL of anhydrous diethyl ether in abeaker, and the resulting solution is transferred to the droppingfunnel. The solution is added dropwise to the reactor flask. After thereaction proceeds, 0.80 mL (15.57 mmol) of concentrated H₂SO₄ is pouredonto ice in a 50 mL beaker. The icy acid is then diluted with distilledwater in a 250 mL beaker, the drying tube is removed from the condenser,and the acid solution is added dropwise to the stirring reactor flask.Once all the magnesium in the flask has reacted, the contents are pouredinto a separating funnel. The organic layer is separated and the aqueouslayer is extracted twice with two 15 mL portions of diethyl ether. Thecombined organic layer is dried over MgSO₄ and the solids are filteredoff of the ethereal solution. To purify the product from the thirdexperiment, the ethereal solution is evaporated to dryness, the flask isthen cooled to room temperature and placed in an ice bath. The crude,solid product is then recrystallized from methanol. The structure andthe molecular weight of the purified product are determined by NMRspectroscopy and GPC, respectively.

In a fourth experiment, the living polymerization of isobutylene wascarried out as follows. Into a flame dried 1 L round bottom flaskequipped with a magnetic stirrer is placed 9.1×10⁻² g (6.7×10⁻⁵ mol)cumyl hydroxyl functionalized αMeSt hexamer initiator, as prepared inthe third experiment, and the flask is evacuated for 30 minutes. Then,198 mL dry hexane and 132 mL dry CH₂Cl₂ are transferred into the flaskby a cannula. Then, 01.18 mL (1.20×10⁻³ mol) TMEDA is added, the systemis then cooled to −80° C., and then isobutylene (18.42 mL, 0.25 mol) isadded. The solution is strongly stirred and the living polymerization ofisobutylene is initiated by the addition of TiCl₄ (1.05 mL, 9.6×10⁻³mol). The reaction is allowed to proceed for 120 minutes. Following thistime period, a sample is withdrawn to be characterized by ¹H NMRspectroscopy and GPC.

In a fifth experiment, the active living intermediate at −80° C., asobtained in the fourth experiment, DTP (0.26 mL, 1.2×10⁻³ mol) andstyrene (6.6 mL, 5.8×10⁻² mol) are added. The polymerization of styreneis allowed to proceed for two hours and is then terminated with theaddition of 10 mL of methanol. The system is warmed to room temperatureand then the solution is concentrated by rotary evaporation andprecipitated into 1 L of methanol. The methanol is then decanted, thepolymer is then dissolved in hexane, and then washed with 5% aqueoussodium bicarbonate and water. The organic phase is dried overnight overmagnesium sulfate, the solids are then removed by filtration throughfine sintered glass, and the solvent is then evaporated by rotaryevaporation. The product, OαMeSt-g-P(IB-b-St), is dried in a vacuum ovenat 50° C. for a two day period. Structural and molecularcharacterization is conducted by ¹H NMR spectroscopy and GPC.

The product, OαMeSt-g-P(IB-b-St), is found to be insoluble in water andalcohols, whereas is found to be soluble in toluene, THF andmethylcyclohexane. OαMeSt-g-P(IB-b-St), is a colorless strong rubberwith good elongation and shows practically or essentially no creep.OαMeSt-g-P(IB-b-St), can be cast, calendared, molded, and extruded. Thetensile strength of OαMeSt-g-P(IB-b-St) is in excess of 20 MPa, and itshows essentially no creep deformation. OαMeSt-g-P(IB-b-St) can be usedin a great variety of industrial TPE applications, whereas linear SIBScannot be used, for example, in automotive and diverse consumerapplication. Its varied use can be attributed to its biocompatibilityand biostability, which also makes OαMeSt-g-P(IB-b-St) useful inimplantable medical devices.

A sixth experiment may be undertaken based on the fact that the PIB andPSt segments in both linear SIBS and OαMeSt-g-P(IB-b-St) are essentiallyidentical. Because of the similar nature of these segments, thesesegmented polymers can be combined to form homogenous blends. Thus,various proportions of linear SIBS and OαMeSt-g-P(IB-b-St) can beblended and the blends can then be used to fine tune the mechanicalproperties and processibility of these blended TPEs. Further, the highcost of forming linear SIBS can be reduced by blending it with lowercost OαMeSt-g-P(IB-b-St). Importantly, the blends are homogeneous as thechemical nature of the PIB segments in both OαMeSt-g-P(IB-b-St) andlinear SIBS are identical.

Linear SIBS may be blended with different amounts, i.e., 10-30% (byweight) of OαMeSt-g-P(IB-b-St). By utilizing a solution blendingtechnique, the two polymers may be dissolved in a common solvent, e.g.,THF, and then the system can be stirred for about 30 minutes so as toensure blending at the molecular level. The blend may then be recoveredby precipitation into methanol and dried to a constant mass in a vacuum.Alternatively, the two segmented polymers can be melt blended in aninternal mixer at a temperature of between about 165 and about 175° C.using shear rates between about 10 and about 100 rpm. For mechanicaltesting, blends can be then compression molded into about 0.6 mm thickfilms in a stainless steel mold at a temperature of between about 165and about 175° C. for 15 minutes.

A seventh experiment may be undertaken in order to increase theoxidation resistance and processibility of commercially available linearpoly(styrene-b-ethylene-b-butylene-styrene) (SEBS, sold under thetradename Kraton™ G by Kraton Co.) Therefore, Kraton™ G (or otherversions of Kraton™) may be blended with OαMeSt-g-P(IB-b-St). As anyversion of Kraton™ contains 20-30% by mass or PSt, it can be blendedwith OαMeSt-g-P(IB-b-St). To achieve desirable effects, any version ofKraton™ may be blended with 10 to 30% (by weight) OαMeSt-g-P(IB-b-St).Blending can be effected by solution or melt blending. Thus, any versionof Kraton™ and various amount of OαMeSt-g-P(IB-b-St) may be dissolved inTHF, that serves to dissolve both constituents, the mixture can bestirred for 30 minutes to ensure blending at the molecular level, andthen recovered by precipitation into methanol. The recovered blend canbe dried to a constant mass in vacuum. Alternatively, the two polymerscan be melt blended in an internal mixer operating at a temperature ofbetween about 165 and about 175° C. under shear rates between about 10and about 100 rpm. For mechanical testing, blends may be compressionmolded into about 0.6 mm thick films in a stainless steel mold at atemperature of between about 165 and about 175° C. for 15 minutes.

An eighth experiment may be undertaken in order to examine what occurswhen the time of the living styrene polymerization is extended beyondcomplete monomer consumption. Recent research has shown that themechanical properties of SIBS increases, without compromising processingproperties, when the time of the living styrene polymerization isextended beyond complete monomer consumption. This effect was shown tobe due to the formation of hyperbranched SIBS by alkylation of PStphenyl groups by live benzyl cations. This effect can also be used toincrease the strength of PSt-g-P(IB-b-St).

Thus to living PIB cations, the preparation of which is detailed abovein the fourth experiment, DtBP (0.26 mL, 1.2×10⁻³ mol) and styrene (6.6mL, 5.8×10⁻³ mol) may be added. The polymerization of styrene may beallowed to proceed in some examples for more than 2 hours, and in otherexamples, for between about 3 and about 6 hours. Subsequently, thepolymerization can be terminated with 10 mL methanol, the system maythen be warmed to room temperature, concentrated by rotary evaporation,and the precipitated into about 1 L methanol. The methanol may then bedecanted, the product then dissolved in hexane, and then the product canbe washed with 5% aqueous sodium bicarbonate and water. The organicphase may be dried over magnesium sulfate overnight, the solids may thenbe separated by filtration through fine sintered glass, and the solventcan be evaporated by rotary evaporation. The product, hyperbranchedOαMeSt-g-P(IB-b-St), may be dried in a vacuum oven at 50° C. for about 2days. Structural, molecular, and mechanical characterizations may beconducted by ¹H NMR spectroscopy, GPC, and universal tensile testingmethods. The results of this experiment would show that if the time ofthe termination of styrene polymerization is extended to 3 to 6 hoursafter the complete consumption of the styrene, processible hyperbranchedOαMeSt-g-P(IB-b-St) can be obtained with increased strength and modulus.

In light of the foregoing, it should be appreciated that the presentinvention significantly advances the art by providing a the novelstructure and synthesis of a star block copolymer and a novel star blockcopolymer-based thermoplastic elastomer that is structurally andfunctionally improved in a number of ways. While particular embodimentsof the invention have been disclosed in detail herein, it should beappreciated that the invention is not limited thereto or therebyinasmuch as variations on the invention herein will be readilyappreciated by those of ordinary skill in the art. The scope of theinvention shall be appreciated from the claims that follow.

What is claimed is:
 1. A star block copolymer comprising a corecomponent including an α-methylstyrene oligomer and arms emanating fromthe core component, wherein the arms are poly(isobutylene-block-styrene)diblock copolymers.
 2. The star block copolymer of claim 1 wherein theα-methylstyrene oligomer contains between 3 and 10 α-methylstyrene unitsand wherein each α-methylstyrene unit has a singlepoly(isobutylene-block-styrene) diblock copolymer arm emanating from astyrene portion of the oligomer.
 3. The star block copolymer of claim 2wherein the number average molecular weight of each polyisobutyleneblock of each poly(isobutylene-block-styrene) diblock copolymer arm isbetween about 30,000 and about 40,000 g/mol and wherein the numberaverage molecular weight of each polystyrene block of eachpoly(isobutylene-block-styrene) diblock copolymer arm is between about12,000 and about 14,000 g/mol.
 4. The star block copolymer of claim 1having a tensile strength of greater than 20 MPa.
 5. The star blockcopolymer of claim 1 having essentially no creep deformation.
 6. Amethod of producing a star block copolymer comprising a core componenthaving an α-methylstyrene oligomer and arms emanating from the corecomponent wherein the arms are poly(isobutylene-block-styrene) diblockcopolymers, the method including the steps of: a. synthesizingα-methylstyrene oligomer; b. acetylating the α-methylstyrene oligomer toform an α-methylstyrene oligomer with acetyl groups; c. converting theacetyl groups to cumyl hydroxide groups; d. undertaking livingcarbocationic polymerization of isobutylene to form polyisobutyleneblocks; and e. undertaking living carbocationic polymerization ofstyrene to form polystyrene blocks at an end of each polyisobutyleneblock to provide the poly(isobutylene-block-styrene) diblock copolymerarms.
 7. The method of claim 6 wherein the step of synthesizing includesadding n-BuLi to α-methylstyrene under strong stirring for a period ofabout 5 hours, followed by the termination of the synthesis by theaddition of methanol.
 8. The method of claim 7 wherein the step ofacetylating includes combining the α-methylstyrene oligomer with AlCl₃,CH₂Cl₂, and CH₃COCl so as to form the α-methylstyrene oligomer withacetyl groups.
 9. The method of claim 8 wherein the step of convertingthe acetyl groups to cumyl hydroxide groups takes place utilizing theGrignard reaction to form a α-methylstyrene oligomer with cumylhydroxide groups.
 10. The method of claim 9 wherein the step ofundertaking living carbocationic polymerization of isobutylene includesadding the α-methylstyrene oligomer with cumyl hydroxide groups toisobutylene and a TiCl₄ coinitiator to form a living intermediate. 11.The method of claim 10 wherein the step of undertaking livingcarbocationic polymerization includes adding styrene to the livingintermediate, followed by the termination of the living carbocationicpolymerization by the addition of methanol.
 12. A thermoplasticelastomer including a plurality of star block copolymers comprising acore component including an α-methylstyrene oligomer and arms emanatingfrom the core component, wherein the arms arepoly(isobutylene-block-styrene) diblock copolymers and wherein at leastsome of the polystyrene blocks of the plurality of star block copolymersare physically crosslinked through aggregation.
 13. Thestar-thermoplastic elastomer of claim 12 wherein the α-methylstyreneoligomer of each of the plurality of star block copolymers containsbetween 3 and 10 α-methylstyrene units and wherein each α-methylstyreneunit has a single poly(isobutylene-block-styrene) diblock copolymer armemanating from a styrene portion of the oligomer.
 14. Thestar-thermoplastic elastomer of claim 13 wherein the number averagemolecular weight of each polyisobutylene block of eachpoly(isobutylene-block-styrene) diblock copolymer arm is between about30,000 and about 40,000 g/mol and wherein the number average molecularweight of each polystyrene block of each poly(isobutylene-block-styrene)diblock copolymer arm is between about 12,000 and about 14,000 g/mol.